1. Field of the Invention
The present invention relates to a wavefront sensor fir measuring wavefront shape of a light beam, and a lens meter and an active optical reflecting telescope using the wavefront sensor.
2. Description of Related Art
Conventionally, a Hartmann wavefront sensor is known as a wavefront sensor for measuring wavefront shape of a light beam. The Hartmann wavefront sensor includes a plate member called the Hartmann plate having multiple small openings regularly formed therein at a constant interval, and an area sensor disposed parallel to the Hartmann plate. A light beam is radiated onto the Hartmann plate from the opposite side of the area sensor. The incident light beam forms a thin pencil or bundle of rays of light as it passes through the openings and creates multiple luminous points on the area sensor according to the number of openings.
When the incident light beam is a plane wave, the interval between the openings in the Hartmann plate and the interval between the luminous points on the area sensor are equal. Even when the interval of the openings and the interval of the luminous points are not equal, the direction of light of the transmitted rays can be calculated from the position of the luminous points on the area sensor, because the distance between the Hartmann plate and the area sensor, and the position of the openings in the Hartmann plate are known Because this direction is equal to a direction normal to the wave plane of incident light beam, wavefront shape of the incident light beam can be measured based on multiple directions of the light.
On the other hand, m order to improve the S/N ratio, each opening of the Hartmann plate is usually equipped with a single focus lens of the same specification, and the area sensor is disposed on the local point of each lens
The conventional wavefront sensor has a problem however that when the incident light beam is a plane wave, the S/N ratio can be sufficiently improved by using a single focus lens and an area sensor located at the focal point of the single focus lens, however, if the incident light beam is not a plane wave, the luminous point on the area sensor becomes blurred and the S/N ratio decreases significantly.
As shown in FIG. 12(a), when a light beam P incident to a lens 2 on the Hartmann plate 1 is a plane wave, light through the lens 2 is collected at one point on an area sensor 3 and creates a luminous point Q1. The luminous point Q1 has a luminous energy distribution indicated by the solid line shown in FIG. 13. However, as shown in FIGS. 12(b) and 12(c), when the incident light beam P is either divergent light or convergent light, light transmitted through the lens 2 does not converge on the area sensor 3 and creates a rather large-size luminous point Q2 or Q3 on the area sensor 3. These luminous points Q2 and Q3 have a luminous energy distribution indicated by the broken line shown in FIG. 13 and thus they are significantly blurred due to the absence of a clear luminous energy difference from the surroundings as demonstrated by the luminous point Q1. Therefore, when the luminous energy received by the area sensor 3 decreases due to the presence of dust or scars on the lenses, it easily affects the luminous points Q2 and Q3, and in the worst case, these points cannot be recognized as a luminous point.
In addition, since the periphery of the blurred luminous point is unclear and extends outwardly far from the center of the luminous point, the adjacent luminous points on the area sensor 3 may touch or overlap each other. To avoid this, it is necessary either to shorten the distance between the Hartmann plate 1 and the area sensor 3, or to enlarge the interval of the openings in the Hartmann plate 1. However, in the former case, for an incident light other than the plans wave, the displacement of the luminous point becomes small, lowering the sensitivity to the displacement. In the latter case, the density of the luminous point becomes small decreasing the measurement point. Thus, the accuracy of the measurement of wavefront shape is deteriorated in both cases.
Especially, when measuring a wavefront greatly distorted from a flat plane, such as a light beam transmitted through a lens, the above-mentioned problems cannot be ignored, and it is absolutely necessary to avoid remarkable blurring of the luminous points so as to improve measurement accuracy.
With the foregoing in view, it is an object of the present invention to provide a wavefront sensor, which is capable of achieving a measurement with high accuracy by avoiding the creation of remarkably blurred luminous points, regardless of wavefront shape of an incident light beam.
Another object of the present invention is to provide a lens meter and a reflection telescope using the wavefront sensor.
To achieve the foregoing objects, the present invention provides in one aspect a wavefront sensor comprising: a plurality of lenses disposed in the same plane and an area sensor which receives a bundle of rays of light collected through each of the lenses as a luminous point so that the wavefront sensor measures wavefront shape of a light beam incident to the lenses based on the position of the luminous points on the area sensor. Each of the lenses comprises a plurality of concentric areas with different focal lengths, and the area sensor is located substantially halfway between a first position in which a plane wave forms an image after passing through one of the concentric areas with a minimum focal length, and a second position in which the plane wave forms an image after passing through another area with a maximum focal length.
In one preferred form of the invention, the respective focal lengths of the concentric areas change stepwise from a central portion toward a peripheral portion of each of the lenses. As an alternative, the respective focal length of the concentric areas change continuously from the central portion toward the peripheral portion of each of the lenses.
Form the manufacturing point of view, it is preferable that the central portion of each lens has the maximum fill length, and the peripheral portion of each lens has the minimum focal length.
The lenses preferably comprise a diffraction optical element.
In another aspect the present invention provides a lens meter in which the wavefront sensor of the foregoing construction is incorporated.
In still another act the present invention provides an active optical reflection telescope using the wavefront sensor of the foregoing construction.